1. Field of the Invention
The invention is a low power heterodyned swept frequency microwave radar system for averaging echoes. The averaged echoes need not be phase correlated. The invention has particular applicability for analyzing distributed targets such as clouds, i.e., a ceilometer.
2. Description of the Prior Art
A prior art search was performed and uncovered the following references:
George, David H., "A Comparison of Currently Used Cloud Height Sensors", Second Symposium, Meteorological Observations and Instrumentation of the American Meteorological Society, Mar. 27-30, 1972, San Diego, CA, p. 223 et seq. provides an overview of the ceilometer art. The paper indicates that the trend in the art is towards lasers and away from microwave radar systems.
Chadwick, R. B. et al, "Measurements Showing the Feasibility for Radar Detection of Hazardous Wind Shear at Airports", AFGL-TR-78-0160, NOAA Final Report Nov. 1, 1976-Feb. 28, 1978, is a Doppler microwave system utilizing phase lock loops and coherent processing, unlike in the present invention. The purpose of Chadwick's device is to measure wind shear at airports.
U.S. Pat. Nos. 3,967,283 and 4,124,848 transmit signals to detect intruders. Unlike the present invention, they do not utilize Fourier transforms.
U.S. Pat. No. 3,680,085 is a swept frequency radar system for obtaining ranging information on a point target. It cannot be used for distributed targets as in the present invention because the patentee obtains his information solely from a single frequency. With distributed targets, different distributed targets could produce the same heterodyned frequency.
U.S. Pat. No. 4,075,630 utilizes a Fast Fourier Transform (FFT) in a pulsed radar system. The use of pulses in a radar system implies high power. The present invention eschews pulses to save cost, to provide greater safety for people and other animals in the near vicinity of the operating system, and for increased reliability.
U.S. Pat. No. 4,028,700 utilizes FFT in a chirp radar system as part of a method for tuning a radar to minimize sidelobes. It has no applicability to the low power swept frequency system of the present invention.
U.S. Pat. No. 4,084,158 utilizes a FFT in a pulsed radar system for lateral resolution data, not for ranging information as in the present invention.
U.S. Pat. No. 4,104,631 uses FFT to obtain Doppler information in a pulsed radar system.
U.S. Pat. No. 3,135,957 is a pulsed radar system, not a continuous wave (CW) system as in the present invention. It needs a fixed reference. It teaches away from the single antenna of the present invention in column 1 lines 26-30 wherein it states, "In frequency-modulated systems in which the peak power is moderate and the transmission and reception are simultaneous, difficulty is encountered in eliminating interference between the reflected signal and direct radiation from the local transmitting system". This problem would be worsened in a CW system. The patent does not use a Fourier transform as in the present invention. The ranging results are obtained just from frequency data, not from amplitude and frequency data as in the present invention.
U.S. Pat. Nos. 3,974,501 and 4,107,681 are FM-CW systems which do not provide for the processing of quasi-incoherent signals, which are obtained from the examination of distributed targets such as clouds.
U.S. Pat. No. 3,181,156 is a chirp radar system signal processing technique. By taking orthogonal Fourier sine and cosine components within computer 22, the patentee utilizes phase information for pulse synthesis. He does not perform a Fourier power transform into the frequency domain to destroy phase information for the purpose of signal averaging, as in the present invention.
U.S. Pat. No. 3,987,285 is a chirp radar system filtering technique which requires two FFT's and reference signals. The present invention does not require two FFT's or reference signals.
Chadwick, R. B., et al "A New Radar For Measuring Winds", Bulletin of the American Meteorological Society, Volume 57 No. 9, pp. 1120-1125, September 1976, is a high power CW-FM radar that measures wind velocity by sensing phase variations from sweep to sweep. The present invention, on the other hand, uses low power and destroys phase information.
Fuller, K. L., "AVOID, A Short-Range High-Definition Radar" Philips Tech. Rev. 32, No. 1, pp. 13-19 (1971) is a radar system which obtains ranging information based only on frequency. It is an analog, not a digital system, and does not use Fourier transforms. On the other hand, the present invention utilizes amplitude and frequency information, is a digital system, and does use Fourier transforms.
Barry, G. H., "A Low-Power Vertical-Incidence Ionosonde", IEEE Transactions on Geoscience Electronics, Vol. GE-9, No. 2, April 1971, pp. 86-89 is a radar ranging technique which is analog, does not use Fourier transforms, and has separate transmitting and receiving antennas. The present invention, on the other hand, is digital, does use Fourier transforms, and in its preferred embodiment utilizes the same J-hook antenna for both transmitting and receiving.
U.S. Pat. No. 3,720,949 is a high power FM-CW radar system that does not use Fourier transforms and does not contemplate the processing of incoherent or quasi-incoherent signals. The present invention, on the other hand, is low power, uses Fourier transforms and does process quasi-incoherent signals such as are produced by distributed targets.
"Category II Final Test Report for Radar Cloud Detecting Set An/TPQ-11 (Production Model)", Technical Report No. ESD-TR-65-7, Dec. 17, 1965, Electronic Systems Command, United States Air Force, describes a pulsed radar ceilometer utilizing extremely high power (100 KW nominal peak power) which makes it much more expensive, dangerous and less reliable than the device of the present invention.
Secondary references are U.S. Pat. Nos. 2,481,034, 2,822,536, 3,603,990, 3,803,609, 3,823,399, 4,044,353, 4,044,355, 4,065,768, 4,092,644, 4,103,300, 4,106,020, 4,106,872, 4,121,889; and Barrick, D. E., "FM/CW Radar Signals and Digital Processing", NOAA Technical Report ERL 283-WPL 26, July 1973. The latter reference addresses the subject of signal processing with respect to FM/CW radar, employing coherent averaging. The reference teaches away from the approach of phaseless averaging as described in the instant specification.